Many skinny folks assume gaining weight is easy because it's "the opposite of dieting," but healthy weight gain (without excess fat) needs just as much planning as weight loss does.
Step 1: Calculate your calorie surplus accurately
Use the calorie calculator to find your daily maintenance calories, then add a moderate surplus (300-500 calories). Too large a surplus leads to rapid, unwanted fat gain.
Step 2: Don't neglect protein
Without enough protein (1.6-2.2g/kg of body weight), a large part of your surplus will turn into fat instead of muscle tissue. Protein is the difference between "gaining weight" and "gaining muscle."
Step 3: Commit to regular resistance training
A calorie surplus without a muscle-building stimulus (weight training) usually turns into fat. Do resistance training 3-4 times a week with progressive overload to ensure your body uses the extra calories to build new muscle.
Step 4: Be patient and track your progress
A healthy gain rate is roughly 0.25-0.5 kg per week. Monitor your weight weekly (not daily, to avoid natural fluctuations), and use the weight gain calculator to adjust your plan based on your actual progress.